The invention is in the field of time-of-flight (TOF) spectrometry. Specifically, the invention relates to TOF spectrometers which include an ion generating source which generates a pulsed ion beam.
Mass spectrometers permit rapid analysis of chemical compounds. A mass spectrometer generally includes a vacuum tube into which a small amount of a gas to be examined is admitted. The gas is ionized, for example by use of a pulsed laser, and the ions are accelerated. The time that it takes an ion to reach a detector is a function of the ratio of the charge q of an ion to the mass m of the ion. Therefore, when ions reach the detector, the ions have separated into bunches corresponding to q/m values. The values of q/m exhibited for a given sample indicate the chemical make-up of the sample.
A reflector can be provided in the flight path of the ions to compensate for the flight times of ions with different energies. Higher energy electrons penetrate deeper into a reflecting field of the reflector and accordingly spend a longer time in the reflector to compensate for the shorter flight times of the higher energy ions in non-field regions. This compensation is called velocity focusing.
General background information on TOF mass spectrometry is provided in "Time-of-Flight Mass Spectrometer with Improved Resolution" by W. C. Wiley et al., appearing in The Review of Scientific Instruments. December 1955, Vol. 26, No. 12, incorporated herein by reference.
TOF mass spectrometry has a major advantage in permitting the simultaneous examination of ions spanning a large mass range. Recently, TOF spectrometry has been used in the mass analysis of cluster beams and the analysis of fragments of large organic molecules, since these applications require examination of ions spanning a large mass range. Analysis of particles expelled in combustion processes is also possible In these fields of application, the low density of the particles to be analyzed make analysis difficult. In addition, in these applications, adequate resolution is difficult to achieve.
Commercially available mass spectrometers usually include potential-shaping wire meshes in both the ion source and the reflector. Wire mesh electrodes are also frequently employed in the detector as well. These meshes reduce the transmission of the ions and cause undesirable secondary effects, such as fragmentation, sputtering of secondary particles, and electron emission by ion impact.
"A High-Resolution Time-of-Flight Mass Spectrometer Using Laser Resonance Ionization" by R. Frey et al., appearing in the Journal For Natural Research. 1985, Issue 12, page 1349 discloses a nonmesh reflector. In the Frey spectrometer, however, the ions must be produced in a very small ionization volume, e.g., 0.1 mm in diameter focus volume, due to the lack of space focusing. This small ionization volume results in poor overall detection sensitivity since sensitivity is proportional to the original ionization volume. The Frey article does not disclose or suggest a non-mesh source. Furthermore, the Frey article does not disclose specific types of potential distributions to be used with the Frey reflector.
The term "space focusing" refers to compensating for differences in times of flight resulting from a finite ionization volume. Space focusing compensates for the finite size of the original ion bunch by concentrating particles within one bunch in the axial direction. Space focusing is different from radial focusing, which results in a smaller beam diameter In conventional instruments, space focusing is achieved with the aid of grids, as discussed in the W. C. Wiley et al. article cited above. These grids reduce the transmission of ions since a portion of the ions collide with the grids.